This invention pertains to separate foreground and background compositing, employing a colored (i.e., blue) backing for the foreground scene.
The so-called chroma-key process for compositing was developed decades ago by engineers of the National Broadcasting Co. in New York City. This was a foreground-background switching system.
Recently, Nakamura et al modified this system to a "soft edge chroma-key", in which edges between foreground and background are purposely blurred. This tends to hide the effects of switching from foreground to background and vice versa. However, detail is lost in this boundary area.
Additionally, Nakamura added a subtraction circuit to eliminate the blue tint at the soft edge.
This system is disclosed in the SMPTE Journal, Vol. 90, No. 2, February 1981, page 107.
All chroma-key (i.e., switching devices), including the Nakamura soft edge device, suffer from two defects; i.e., loss of sharp edges on objects that have such edges, and loss of fine detail at the edges. The soft edge frequency response must be about half that of the capability of the foreground camera.
Vlahos U.S. Pat. No. 3,595,987 introduces the concept of developing a control signal that is proportional to the brightness and visibility of the colored backing, also of controlling the level of the background scene as a linear function of the amplitude of the control signal, and also of eliminating the blue backing by limiting that video signal amplitude to a maximum that is represented by the amplitude of one of the other primary colors.
The patent does not disclose removal of residual contaminating colors by subtraction, the rejection of foreground luminance because of secondary illumination from the backing, of encoded video, nor defining mixing as additive or non-additive.
Vlahos U.S. Pat. No. 4,007,487 introduces a (B-G)+(G-R) control signal; the (G-R) term permits the reproduction of blue eyes. A color ratio is established to distinguish the colored backing from the subject even with a backing of non-uniform brightness.
The patent does not disclose removal of contaminating colors by subtraction, nor of removing or retaining secondary luminance on the subject. Nor does it disclose encoded video, or specify additive or non-additive mixing.
Vlahos U.S. Pat. No. 4,100,569 introduces full rejection of the colored backing by subtracting its red, green and blue components in accordance with a control signal that varies as a function of the intensity of these colors as they appear in the backing area.
A control for the color magenta is provided. Control E.sub.c is described, but without the -K(1-B) term.
An encoded color signal is not disclosed, nor is mixing of foreground and background video signal specified as additive or non-additive.